The present invention relates to a gas turbine system for generating shaft power. More specifically, the present invention relates to a hot gas manifold for a gas turbine system having a plurality of external topping combustors for heating gas from a pressurized fluidized bed combustor ("PFBC").
The high efficiency, low capital cost and short lead time of gas turbine based systems make them particularly attractive to electric utilities as a means for producing electrical power. However, traditionally, gas turbine operation has been limited to expensive, sometimes scare, fuels--chiefly distillate oil and natural gas. As a result of the ready availability and low cost of coal, considerable effort has been expended toward developing a gas turbine system for generating electrical power that can utilize coal as its primary fuel. One area in which these efforts have focused concerns systems in which the combustion of coal is carried out in a pressurized fluidized bed combustor "PFBC."
In one of the simplest gas turbine/PFBC power plant arrangements, ambient air, compressed in the compressor section of the gas turbine, serves to fluidize the bed and provides combustion air for the PFBC. After combustion in the PFBC, the air, now at a high temperature and vitiated by the products of combustion and entrained particulate matter, is exhausted from the PFBC. The air then flows through a cyclone separator wherein much of the particulate matter is removed. The air is then directed to the turbine section of the gas turbine where it is expanded, thereby producing useful shaft power. After expansion, the vitiated air exhausted from the turbine is vented to atmosphere.
The thermodynamic efficiency of such a system is poor, however, due to the need to limit the bed temperature, and hence the temperature of the air entering the turbine section, in order to optimize capture of the sulfur in the coal and avoid carryover of harmful alkali vapors into the turbine section. This is in contrast to modern conventional gas or liquid fuel fired gas turbines, which can operate with turbine inlet gas temperatures as high as 1425.degree. C. (2600.degree. F.). As is well known in the art, increasing the temperature of the gas entering the turbine section increases the power output and efficiency of the gas turbine.
Hence, to achieve maximum efficiency, it has been proposed to employ a separate topping combustor--that is, a combustor external to the gas turbine and the PFBC--to raise the temperature of the air leaving the PFBC to the temperature required for maximum efficiency in the turbine. Although the topping combustor may be fired on oil or natural gas, to maximize coal utilization, the addition of a pyrolysis treatment operation (carbonizer) to the system has been proposed. The carbonizer converts coal to a low BTU gas and a solid, carbonaceous char. The low BTU gas is burned in the topping combustor and the char is burned in the PFBC.
In addition, it may be desirable to utilize more than one topping combustor--for example, in order to allow the gas turbine to remain in operation while one of the topping combustors is being repaired. However, in such situations, the hot gas from the topping combustor(s) remaining in service must be distributed as evenly as possible around the turbine section to avoid creating an asymmetric flow pattern within the turbine section on account of the inoperative topping combustor. Accordingly, it would be desirable to provide a manifold system for distributing hot gas from each of a plurality of topping combustors to the inlet to the turbine section of a gas turbine so that the hot gas from each individual combustor is symmetrically distributed around the turbine inlet.